Dr. Didier Long "Strain hardening of glassy polymers: theory and simulation"
Laboratoire Polymère et Matériaux Avancés (LPMA), Solvay, Saint-Fons, France
May 24, 2017
from 02:15 PM to 03:00 PM
|Where||Seminarraum A, FMF, Stefan-Meier-Str. 21, Freiburg|
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Experiments show that glassy polymers submitted to an applied stress undergo yield at deformations of a few percent and stresses of some 10 MPa, followed by a slow drop in stressunder plastic deformation corresponding to the strain-softening regime. Yield behavior is often described by using the phenomenological Eyring model, according to which stress biases the motion. This model requires the introduction of a so-called activation volume of order typically 1 nm3 without clear interpretation. Upon increasing the deformation, some polymers of high molecular weight display an increase of stress in the large amplitude regime of deformation. The typical slope of stress versus strain in this regime, GR, is of order 107 – 108 Pa well below Tg. Classical theories involving the entropic response of the rubbery network cannot explain such a high value. GR is also found to increase upon cooling.
Regarding the onset of plastic flow and the stress softening regime, we propose that the elastic energy stored in the volume x3 of dynamical heterogeneities effectively reduces the free energy barriers present for internal relaxation. It allows for calculating yield stresses of order a few 10 MPa which are consistent as compared to experimental data without additional adjustable parameters than the scale of dynamical heterogeneities measured and theoretically calculated in other contexts. Regarding the strain hardening regime, and following Chen and Schweitzer, Phys. Rev. Lett. 2009, we assume that local deformation induces a reduction of mobility at the scale of dynamical heterogeneities, by orienting monomers in the drawing direction. We assume that consequent strengthening of monomer-monomer interactions results in a local increase of the glass transition temperature.
The model is then solved in 3D with a spatial resolution corresponding to the scale of a few nanometers of dynamical heterogeneities. Simulation results are in agreement with experimental data, such as the elastic modulus (G’∼1 GPa Pa and its temperature dependence), the yield stress and the yield behavior (strain softening), and the strain hardening regime (GR∼10 MPa) with its temperature dependence, and its dependence on reticulation density.
Ref: Luca Conca PhD thesis, Lyon, 2016
invited by Günter Reiter